I've been on all four of these flights in economy over the years and from my point of view, I tend to get over the jetlag earlier if I fly the earlier LHR-HKG BA31 that leaves around 6pm vs the later one which leaves around 9pm. Though sometimes I will choose the later flight as it allows me to work a full day in London and save on annual leave.

Both flights used to be operated by 747s but now that the product is A380 vs 777, I definitely prefer the A380 on the Upper Deck. Love the side storage bins and the cabin seems more quiet and private. Its also 2-4-2 on the Upper Deck A380 vs 3-3-3 on the 777

On the HKG-LHR as you say, there is not much difference in the flight times so I would choose based on equipment, so the A380 wins out for me.